aircraft instruments.doc

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The Instrument Landing System (ILS)

Instrument Landing System (ILS) facilities are a highly accurate and dependablemeans of navigating to the runway in IFR conditions. When using the ILS, the pilotdetermines aircraft position primarily by reference to instruments. The ILS consistsof:

a. the localizer transmitter;b. the glide path transmitter;c. the outer marker (can be replaced by an NDB or other fix);d. the approach lighting system.

ILS is classified by category in accordance with the capabilities of the groundequipment. Category I ILS provides guidance information down to a decision height(DH) of not less than 200 ft. Improved equipment (airborne and ground) provide forCategory II ILS approaches.

A DH of not less than 100 ft. on the radar altimeter is authorized for Category IIILS approaches.

The ILS provides the lateral and vertical guidance necessary to fly a precisionapproach, where glide slope information is provided. A precision approach is anapproved descent procedure using a navigation facility aligned with a runway where glideslope information is given. When all components of the ILS system are available,including the approved approach procedure, the pilot may execute a precision approach.

Localizer

1. GROUND EQUIPMENT: The primary component of the ILS is the localizer, whichprovides lateral guidance. The localizer is a VHF radio transmitter and antenna systemusing the same general range as VOR transmitters (between 108.10 MHz and 111.95MHz). Localizer frequencies, however, are only on odd-tenths, with 50 kHz spacingbetween each frequency. The transmitter and antenna are on the centerline at the oppositeend of the runway from the approach threshold.

The localizer back course is used on some, but not all ILS systems. Where the backcourse is approved for landing purposes, it is generally provided with a 75 MHz back

marker facility or NDB located 3 to 5 NM from touchdown. The course is checkedperiodically to ensure that it is positioned within specified tolerances.

2. SIGNAL TRANSMISSION: The signal transmitted by the localizer consists of twovertical fan-shaped patterns that overlap, at the center (see ILS Localizer Signal Patternfigure, below). They are aligned with the extended centerline of the runway. The rightside of this pattern, as seen by an approaching aircraft, is modulated at 150 Hz and is


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called the "blue" area. The left side of the pattern is modulated at 90 Hz and is called the"yellow" area. The overlap between the two areas provides the on-track signal.

The width of the navigational beam may be varied from approximately 3 to 6,with 5 being normal. It is adjusted to provide a track signal approximately 700 ft wide at

the runway threshold. The width of the beam increases so that at 10 NM from thetransmitter, the beam is approximately one mile wide.

The localizer is identifiedby an audio signal superimposed on the navigationalsignal. The audio signal is a two-letter identification preceded by the letter "I", e.g., " I-OW ".

The reception range of the localizer is at least 18 NM within 10 degrees of theon-track signal. In the area from l0 to 35 of the on-track signal, the reception range isat least 10 NM. This is because the primary strength of the signal is aligned with therunway centerline.

3. LOCALIZER RECEIVER: The localizer signal is received in the aircraft by a localizerreceiver. The localizer receiver is combined with the VOR receiver in a single unit. Thetwo receivers share some electronic circuits and also the same frequency selector, volumecontrol, and ON-OFF control.

The localizer signal activates the vertical needle called the track bar (TB).Assuming a final approach track aligned north and south (see ILS Localizer SignalPattern figure, above), an aircraft east of the extended centerline of the runway (position1) is in the area modulated at 150 Hz. The TB is deflected to the left. Conversely, if theaircraft is in the area west of the runway centerline, the 90 Hz signal causes the TB todeflect to the right (position 2). In the overlap area, both signals apply a force to theneedle, causing a partial deflection in the direction of the strongest signal. Thus, if anaircraft is approximately on the approach track bur slightly to the right, the TB is


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deflected slightly to the left. This indicates that a correction to the left is necessary toplace the aircraft in precise alignment.

At the point where the 90 Hz and 150 Hz signals are of equal intensity, the TB iscentered, indicating that the aircraft is located precisely on the approach track (position

3).

When the TB is used in conjunction with the VOR, full scale needle deflectionoccurs 10 either side of the track shown on the track selector. When this same needle isused as an ILS localizer indicator, full-scale needle deflection occurs at approximately2.5 from the center of the localizer beam.

Thus the sensitivity of the TB is approximately four times greaterwhen used as alocalizer indicator as opposed to VOR navigation.

In the localizer function, the TB does not depend on a correct track selector

setting in Most cases; however, the pilot should set the track selector for the approachtrack as a reminder of the final approach.

When an OFF flag appears in front of the vertical needle, it indicates that thesignal is too weak, and, therefore, the needle indications arc unreliable. A momentaryOFF flag, or brief TB needle deflections, or both, may occur when obstructions or otheraircraft pass between the transmitting antenna and the receiving aircraft.

Glide Slope Equipment

1. TRANSMITTER: The glide slope provides vertical guidance to the pilot during theapproach. The ILS glide slope is produced by a ground-based UHF radio transmitter andantenna system, operating at a range of 329.30 MHz to 335.00 MHz, with a 50 kHzspacing between each channel. The transmitter is located 750 to 1,250 feet (ft) down therunway from the threshold, offset 400 to 600 ft from the runway centerline. Monitored toa tolerance of 1/2 degree, the UHF glide path is "paired" with (and usuallyautomatically tuned by selecting) a corresponding VHF localizer frequency.

Like the localizer, the glide slope signal consists of two overlapping beamsmodulated at 90 Hz and 150 Hz (see Glide Slope Signal Pattern figure, below). Unlike

the localizer, however, these signals are aligned above each other and are radiatedprimarily along the approach track. The thickness of the overlap area is 1.4 or .7 aboveand .7 below the optimum glide slope.


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This glide slope signal may be adjusted between 2 and 4.5 above a horizontalplane. A typical. adjustment is 2.5 to 3, depending upon such factors as obstructionsalong the approach path and the runway slope.

False signals may be generated along the glide slope in multiples of the glide pathangle, the first being approximately 6 degrees above horizontal. This false signal will bea reciprocal signal (i.e. the fly up and fly down commands will be reversed). The falsesignal at 9 will be oriented in the same manner as the true glide slope. There are no falsesignals below the actual slope. An aircraft flying according to the published approachprocedure on a front course ILS should not encounter these false signals.

2. SIGNAL RECEIVER: The glide slope signal is received by a UHF receiver in theaircraft. In modern avionics installations, the controls for this radio are integrated withthe VOR controls so that the proper glide slope frequency is tuned automatically whenthe localizer frequency is selected.

The glide slope signal activates the glide slope needle, located in conjunction withthe TB (see Glide Slope Signal Pattern figure, above). There is a separate OFF flag inthe navigation indicator for the glide slope needle. This flag appears when the glide slopesignal is too weak. As happens with the localizer, the glide slope needle shows fulldeflection until the aircraft reaches the point of signal overlap. At this time, the needleshows a partial deflection in the direction of the strongest signal. When both signals are

equal, the needle centers horizontally, indicating that the aircraft is precisely on the glidepath.

The pilot may determine precise location with respect to the approach path byreferring to a single instrument because the navigation indicator provides both verticaland lateral guidance.In the Glide Slope Signal Pattern figure, above, position 1, showsboth needles centered, indicating that the aircraft is located in the center of the approachpath. The indication at position 2 tells the pilot to fly down and left to correct the


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approach path. Position 3 shows therequirements to fly up and right toreach the proper path. With 1.4 ofbeam overlap, the area isapproximately 1,500 ft thick at 10

nautical miles (NM), 150 ft at l NM,and less than one foot at touchdown.

The apparent sensitivity of theinstrument increases as the aircraft nears the runway. The pilot must monitor it carefullyto keep the needle centered. As said before, a full deflection of the needle indicates thatthe aircraft is either high or low but there is no indication of how high or low.

ILS Marker Beacons

l . GENERAL: Instrument landing system marker beacons provide information ondistance from the runway by identifying predetermined points along the approach track.These beacons are low-power transmitters; that operate at a frequency of 75 MHz with 3W or less rated power output. They radiate an elliptical beam upward from the ground. Atan altitude of 1,000 ft, the beam dimensions are 2,400 ft long and 4,200 ft wide. At higheraltitudes, the dimensions increase significantly.

2. OUTER MARKER (OM): The outer marker (if installed) is located 3 1/2 to 6 NMfrom the threshold within 250 ft of the extended runway centerline. It intersects the glideslope vertically at approximately 1,400 ft above runway elevation. It also marks the

approximate point at which aircraft normally intercept the glide slope, and designates thebeginning of the final approach segment. The signal is modulated at 400 Hz, which is anaudible low tone with continuous Morse code dashes at a rate of two dashes per second.The signal is received in the aircraft by a 75 MHz marker beacon receiver. The pilotbears a tone over the speaker or headset and sees a blue light that flashes insynchronization with the aural tone (seethe Marker Beacon Lights figure, above right).Where geographic conditions prevent the positioning of an outer marker, a DME unitmay be included as part of the ILS system to provide the pilot with the ability to make apositive position fix on the localizer. In most ILS installations, the OM is replaced by anNDB.

3. MIDDLE MARKER (MM): Middle markers have been removed from all ILS facilitiesin Canada bur are still used in the United States. The middle marker is located.approximately .5 to .8 NM from the threshold on the extended

runway centerline. The middle marker crosses the glide slope at approximately 200 to250 ft above the runway elevation and. is near the missed approach point for the ILSCategory l approach.


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4. BACK MARKER (BM): The back course marker (BM), if installed, is normallylocated on the localizer back course approximately four to six miles from the runwaythreshold. The BM low pitched tone (400 Hz) is beard as a series of dots. It illuminatesthe aircraft's white marker beacon light. An NDB or DME fix can also be used and inmost locations replace the BM.

Lighting systems

1. GENERAL: Various runway environment lighting systems serve as integral parts ofthe ILS system to aid the pilot in landing. Any or all of the following lighting systemsmay be provided at a given facility: approach light system (ALS), sequenced flashinglight (SFL), touchdown zone lights (TDZ) and centerline lights (CLL-required forCategory II [Cat II] operations.)

2. RUNAWAY VISIBILITY MEASUREMENT: In order to land, the pilot must be ableto see appropriate visual aids not later than the arrival at the decision height (DH) or themissed approach point (MAP).

Until fairly recently, the weather observer simply "peered into the murk", tryingto identify landmarks at known distances from the observation point. This method israther inaccurate; therefore, instrumentation was developed to improve the observer'scapability.

The instrument designed to provide visibility information is called atransmissometer. It is normally located adjacent to a runway. The light source (see the

Transmissometer figure, on the right) is separated from the photo-electric cell receiverby 500 to, 700 ft. The receiver, connected to the instrument readout in the airport tower,senses the reduction in the light level between it and the light source caused by increasingamounts of particulate matter in the air. In this way the receiver measures the relativetransparency or opacity of the air. The readout is calibrated in feet of visibility and iscalled runway visual range (RVR).

3. RUNAWAY VISUAL RANGE (RVR): The RVR is the maximum distance in thedirection of take-off or landing at which the runway or the specified light or markersdelineating it can be seen from a height corresponding to the average eye-level of pilotsat touchdown.

Runway visual range readings usually are expressed in hundreds of feet. Forexample, "RVR 24" means that the visual range along the runway is 2,400 ft. In weatherreports, RVR is reported in a code: R36/4000 FT/D; meaning RVR for Runway 36 is4000 ft and decreasing. Because visibility may differ from one runway to another, theRVR value is always given for the runway where the equipment is located. At times,visibility may even vary at different points along the same runway due to a local


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condition such as a fog bank, smoke, or a line of precipitation. For this reason, additionalequipment may be installed for the departure end and mid-point of a runway.

Runway visual range reports are intended to indicate bow far the pilot can seealong the runway in the touchdown zone; however, the actual visibility at other points

along the runway may differdue to the siting of the transmissometer. The pilot shouldtake this into, account when making decisions based on reported RVR.

Runway visual range is not reported unless the prevailing visibility is less thantwo miles or the RVR is 6,000 ft or less. This is so because the equipment cannotmeasure RVR above 6,000 ft. When it is reported, RVR can be used as an aid to pilots indetermining what to expect during the final stages of an instrument approach. Instrumentapproach charts state the advisory values of visibility and RVR.

Runway visual range information is provided to the ATC arrival control. sector,the PAR position, and the control tower or FSS. It is passed routinely to the pilot when

conditions warrant. RVR information may be included in aviation weather reports.

Ground visibility will continue to be reported and used in the application of take-off and landing minima. At runways with a transmissometer and digital readoutequipment or other suitable means, RVR is used in lieu of prevailing visibility indetermining the visibility minima unless affected by a local weather phenomenon of shortduration.

The normal RVR reading is based on a runway light setting of strength 3. If thelight settings are increased to strength 4 or 5, it causes a relative increase in the RVRreading. No decrease in the RVR reading is evident for light settings of less than setting

3. Pilots shall be advised when the runway light setting is adjusted to 4 or 5. If the RVRfor a runway is measured at two locations, the controller identifies the touchdownlocation as "ALFA and the mid runway location as "Bravo".

In all cases, the pilot can request a light setting suitable for his or herrequirements. When more than one aircraft is conducting an approach, the pilot of thesecond aircraft may request a change in the light setting after the first aircraft hascompleted its landing.

Because of the complex equipment requirements, RVR usually is only available atmore active airports and not necessarily for all runways. If RVR equipment is not

available or temporarily out of service for a given runway, the pilot uses the observermethod to provide visibility information. In this case, the visibility is expressed as milesor fractions of a mile. The relationship between RVR values and visibility is shownbelow.


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NDBs At Marker Beacon Sites

Additional aids may be available to assist the pilot in reaching the final approach fix. Oneof these aids is the NDB which can be co-located with or replace the outer marker (OM)or back marker (BM). It is a low-frequency non-directional beacon with a transmitting

power of less than 25 watts (W) and a frequency range of 200 kilohertz (kHz) to 415kHz. The reception range of the radio beacon is at least 15 nautical miles (NM). In anumber of cases an en route NDB is purposely located at the outer marker so that it mayserve as a terminal as well as an en route facility.


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Microwave Landing System (MLS)

The time-referenced scanning beam Microwave Landing System (MLS) hasbeen adopted by ICAO as the standard precision approach system to replace ILS. MLSprovides precision navigation guidance for alignment and descent of aircraft on approach

to a landing by providing azimuth, elevation and distance. The system may be dividedinto five functions:

1. Approach azimuth;2. Back azimuth;3. Approach elevation;4. Range; and5. Data communications.

With the exception of DME, all MLS signals are transmitted on a singlefrequency through time sharing. Two hundred channels are available between 5031 and

5090.6 Megahertz (MHz). By transmitting a narrow beam which sweeps across thecoverage area at a fixed scan rate, both azimuth and elevation may be calculated by anairborne receiver which measures the time interval between sweeps. For the pilot, theMLS presentation will be similar to ILS with the use of a standard CDI or multi-functiondisplay,

ILS Limitations

The Instrument Landing System (ILS) has served as the standard precision approach andlanding aid for the last 40 years. During this time it has served well and has undergone anumber of improvements to increase its performance and reliability. However, in relation

to future aviation requirements, the ILS has a number of basic limitations:

1. site sensitivity and high installation costs;2. single approach path;3. multi path interference; and4. channel limitations - 40 channels only.

MLS Advantages

As previously mentioned, ILS has limitations which prohibit or restrict its use inmany circumstances. MLS not only eliminates these problems; but also offers many

advantages over ILS including:

1. elimination of ILS/FM broadcast interference problems;2. provision of ail-weather coverage up to 60 degrees from runway centerline, from

0.9 degree to 15 degrees in elevation, and out of 20 nautical miles (NM);3. capability to provide precision guidance to small landing areas such as roof-top

heliports;


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4. continuous availability of a wide range of glide paths to accommodate STOL andVTOL aircraft and helicopters;

5. accommodation of both segments and curved approaches;6. availability of 200 channels - five times more than ILS;7. potential reduction of Category I (CAT l) minimums;

8. improved guidance quality with fewer flight path corrections required;9. provision of back-azimuth for missed approaches and departure guidance;10. elimination of service interruptions caused by snow accumulation; and11. lower site preparation, repair, and maintenance costs.

Approach Azimuth Guidance

The approach azimuth antenna normally provides a lateral coverage of 40 either side ofthe center of scan (see MLS Azimuth and Elevation Coverage figure, below). Coverageis reliable to a minimum of 20 NM from the runway threshold and to a height of 20,000feet (ft). The antenna is normally located about 1000 feet beyond the end of the runway.

Back Azimuth Guidance

The back azimuth antenna provides lateral guidance for missed approach anddeparture navigation. The back azimuth transmitter is essentially the same as theapproach azimuth transmitter. However, the equipment transmits at a somewhat lowerdata rate because the guidance accuracy requirements are not as stringent as for thelanding approach. The equipment operates on the same frequency as the approachazimuth bur at a different time in the transmission sequence. On runways that have MLSapproaches on both ends, the azimuth equipment can be switched in their operation fromthe approach azimuth to the back azimuth and vice versa. The MLS Azimuth and

ElevationCoverage figure, below,shows MLS azimuth coverage volumes.


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Elevation Guidance

The elevation station transmits signals on the same frequency as the azimuthstation. The elevation transmitter is normally located about 400 ft from the side of therunway between the threshold and the touchdown zone. The MLS Azimuth and

Elevation Coverage figure, above, shows coverage volumes for the MLS elevationsignal. It allows for a wide range of glide path angles selectable by the pilot.

Range Guidance

Range guidance, consistent with the accuracy provided by the azimuth andelevation stations, is provided by the MLS precision DME (DME/P). DME/P has anaccuracy of +100 ft as compared, with + 1200 ft accuracy of the standard. DME system.In the future it may be necessary to deploy DME/P with modes which could beincompatible with standard airborne DME receivers.

Data Communications

The azimuth ground station includes data transmission in its signal format whichincludes both basic and auxiliary data. Basic data may include approach azimuth trackand minimum glide path angle. Auxiliary data may include additional approachinformation such as runway condition, wind-shear or weather.


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Altimeter

An altimeter is an instrument used to measure the altitude of an object above afixed level. The measurement of altitude is called altimetry, which is related to the termbathymetry, the measurement of depth underwater.

Pressure altimeter

Digital barometric pressure sensor for altitude measurement in consumer electronic

applications

A pressure altimeter (also called barometric altimeter) is the altimeter found inmost aircraft. In it, an aneroid barometermeasures the atmospheric pressure from a staticport outside the aircraft. Air pressure decreases with an increase of altitudeapproximately 100 millibars per 800 meters or one inch of mercury per 1000 feet nearsealevel.

The altimeter is calibrated to show the pressure directly as an altitude above meansea level, in accordance with a mathematical model defined by the International StandardAtmosphere (ISA). Older aircraft used a simple aneroid barometer where the needle

made less than one revolution around the face from zero to full scale. Modern aircraft usea "sensitive altimeter" which has a primary needle that makes multiple revolutions, andone or more secondary needles that show the number of revolutions, similar to a clockface. In other words, each needle points to a different digit of the current altitudemeasurement.

Diagram showing the internal components of the sensitive aircraft altimeter.

On a sensitive altimeter, the sea level reference pressure can be adjusted by asetting knob. The reference pressure, in inches of mercury in Canada and the US andmillibars (orhectopascals) elsewhere, is displayed in theKollsman Window, visible at the
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right side of the aircraft altimeter shown here. This is necessary, since sea level referenceatmospheric pressure varies with temperature and the movement ofpressure systems inthe atmosphere.

In aviation terminology, the regional or local air pressure at mean sea level (MSL)

is called the QNH or "altimeter setting", and the pressure which will calibrate thealtimeter to show the height above ground at a given airfield is called the QFE of thefield. An altimeter cannot, however, be adjusted for variations in air temperature.Differences in temperature from the ISA model will, therefore, cause errors in indicatedaltitude.

Kollsman-type barometric aircraft altimeter as used in North America displaying analtitude of 80 feet.

The calibration formula for an altimeter, up to 36,090 feet (11,000 m), can be written as:

where h is the indicated altitude in feet, Pis the static pressure and Pref is the referencepressure (use same units for both). This is derived from thebarometric formula using thescale height for the troposphere.

Radar altimeter

A radar altimetermeasures altitude more directly, using the time taken for a radio signalto reflect from the surface back to the aircraft. The radar altimeter is used to measureheight above ground level during landing in commercial and military aircraft. Radar

altimeters are also a component of terrain avoidance warning systems, warning the pilotif the aircraft is flying too low, or if there is rising terrain ahead. Radar altimetertechnology is also used in terrain-following radarallowing fighter aircraft to fly at verylow altitude.
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Other Modes of Transport

The altimeter is an instrument optional in off-road vehicles to aid in navigation. Some

high-performance luxury cars which were never intended to leave paved roads, such asthe Duesenberg in the 1930s, have also been equipped with an altimeter.

Mountaineers use wrist-mounted barometric altimeters when on high-altitudeexpeditions, as do skydivers.

Measuring air pressure

The local atmospheric pressure orambient pressure is displayed in the Kollsman windowof a sensitive altimeter, when it is adjusted to read zero altitude.

Satellites

This graph shows the rise in global sea level measured by theNASA/CNES oceanaltimeter mission TOPEX/Poseidon (on the left) and its follow-on mission Jason-1.Image credit: University of Colorado

A number of satellites use advanced dual-band radaraltimeters to measure heightfrom a spacecraft. That measurement, coupled with orbital elements (possibly augmentedby GPS), enables determination of the terrain. The two different wavelengths of radio

waves used permit the altimeter to automatically correct for varying delays in theionosphere.

Spaceborne radar altimeters have proven to be superb tools for mapping ocean-surface topography, the hills and valleys of the sea surface. These instruments send amicrowave pulse to the oceans surface and time how long it takes to return. Amicrowave radiometer corrects any delay that may be caused by water vapor in theatmosphere. Other corrections are also required to account for the influence of electrons
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in the ionosphere and the dry air mass of the atmosphere. Combining these data with theprecise location of the spacecraft makes it possible to determine sea-surface height towithin a few centimetres (about one inch). The strength and shape of the returning signalalso provides information on wind speed and the height of ocean waves. These data areused in ocean models to calculate the speed and direction of ocean currents and the

amount and location of heat stored in the ocean, which, in turn, reveals global climatevariations.
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VHF Omni-directional Range (VOR)

VHF Omni-directional Radio Range (VOR) is a type ofradio navigation systemforaircraft. VORs broadcast a VHFradio composite signal including the station'smorsecode identifier (and sometimes a voice identifier), and data that allows the airbornereceiving equipment to derive a magnetic bearing from the station to the aircraft(direction from the VOR station in relation to the Earth's magnetic North at the time ofinstallation). VOR stations in areas of magnetic compass unreliability are oriented withrespect to True North. This line of position is called the "radial" from the VOR. Theintersection of two radials from different VOR stations on a chart allows for a "fix" orapproximate position of the aircraft.

Developed from earlier Visual-Aural Range (VAR) systems, the VOR was

designed to provide 360 courses to and from the station selectable by the pilot. Earlyvacuum tube transmitters with mechanically-rotated antennas were widely installed in the1950s, and began to be replaced with fully solid-state units in the early 1960s. Theybecame the major radio navigation system in the 1960s, when they took over from theolder radio beacon and four-course (low/medium frequency range) system. Some of theolder range stations survived, with the four-course directional features removed, as non-directional low or medium frequency radio beacons (NDBs).

The VOR's major advantage is that the radio signal provides a reliable line(radial) from the station which can be selected and followed by the pilot. A worldwideland-based network of "air highways", known in the US as Victor Airways (below 18,000

feet) and "jet routes" (at and above 18,000 feet), was set up linking VORs. An aircraftcould follow a specific path from station to station by tuning the successive stations onthe VOR receiver, and then either following the desired course on a Radio MagneticIndicator, or setting it on a conventional VOR indicator (shown below) or a HorizontalSituation Indicator (HSI, a more sophisticated version of the VOR indicator) and keepinga course pointer centered on the display.

VORs provide considerably greater accuracy and reliability than NDBs due to acombination of factors in their construction -- specifically, less course bending aroundterrain features and coastlines, and less interference from thunderstorms. Although VORtransmitters were more expensive to install and maintain, today VOR has almost entirelyreplaced the low/medium frequency ranges and beacons in civilian aviation, and is nowin the process of being supplanted by the Global Positioning System (GPS). Because theywork in the VHF band, VOR stations rely on "line of sight" -- if the transmitting antennacould not be seen on a perfectly clear day from the receiving antenna, a useful signalcannot be received. This limits VOR (and DME) range to the horizon -- or closer ifmountains intervene. This means that an extensive network of stations is needed toprovide reasonable coverage along main air routes. The VOR network is a significant
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cost in operating the current airway system, although the modern solid state transmittingequipment requires much less maintenance than the older units.

See the text for details on the four components of the VOR Indicator.

The digital indicator is a separate gauge used on the Nav Trainer Panel.

The VOR display has four elements:

A. A Rotating Course Card, calibrated from 0 to 360, which indicates the VORbearing chosen as the reference to fly TO or FROM. Here, the 345 radial hasbeen set into the display. This VOR gauge also digitally displays the VORbearing, which simplifies setting the desired navigation track.

B. The Omni Bearing Selector, or OBS knob, used to manually rotate the coursecard.

C. The CDI, or Course Deviation Indicator. This needle swings left or rightindicating the direction to turn to return to course. When the needle is to the left,turn left and when the needle is to the right, turn right, When centered, the aircraftis on course. Each dot in the arc under the needle represents a 2 deviation fromthe desired course. This needle is more-frequently called the left-right needle,with the CDI term quickly forgotten after taking the FAA written exams. Here,the pilot is doing well, and is dead-on courseor maybe lazy and with theautopilot activated in the "NAV" mode.

D. The TO-FROM indicator. This arrow will point up, or towards the nose of theaircraft, when flying TO the VOR station. The arrow reverses direction, pointsdownward, when flying away FROM the VOR station. A red flag replaces theseTO-FROM arrows when the VOR is beyond reception range, has not beenproperly tuned in, or the VOR receiver is turned off. Similarly, the flag appears if


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the VOR station itself is inoperative, or down for maintenance. Here, the aircraftis flying TO the station.

Operation

VORs are assigned radio channels between 108.0 MHz (megahertz) and 117.95MHz (with 50 kHz spacing); this is in the VHF (very high frequency) range.

The VOR system uses the phase relationship between a reference-phase and arotating-phase signal to encode direction. The carrier signal is omni-directional andcontains an amplitude modulated (AM) station Morse code or voice identifier. Thereference 30 Hz signal isfrequency modulated (FM) on a 9960 Hz sub-carrier. A second,amplitude modulated (AM) 30 Hz signal is derived from the rotation of a directionalantenna array 30 times per second. Although older antennas were mechanically rotated,current installations scan electronically to achieve an equivalent result with no movingparts. When the signal is received in the aircraft, the two 30 Hz signals are detected and

then compared to determine the phase angle between them. The phase angle is equal tothe direction from the station to the airplane, in degrees from local magnetic north, and iscalled the "radial."

This information is then fed to one of three common types of indicators:

1. The typical light-airplane VOR indicator is shown in the accompanyingillustration. It consists of a knob to rotate an "Omni Bearing Selector" (OBS), andthe OBS scale around the outside of the instrument, used to set the desired course.A "course deviation indicator" (CDI) is centered when the aircraft is on theselected course, or gives left/right steering commands to return to the course. An

"ambiguity" (TO-FROM) indicator shows whether following the selected coursewould take the aircraft to, or away from the station.2. A Horizontal Situation Indicator (HSI) is considerably more expensive and

complex than a standard VOR indicator, but combines heading information withthe navigation display in a much more user-friendly format.

3. A Radio Magnetic Indicator (RMI) was developed previous to the HSI, andfeatures a course arrow superimposed on a rotating card which shows theaircraft's current heading at the top of the dial. The "tail" of the course arrowpoints at the current radial from the station, and the "head" of the arrow points atthe reciprocal (180 degrees different) course to the station.

In many cases the VOR stations have colocated DME (Distance MeasuringEquipment) or military TACAN (TACtical Air Navigation -- which includes both thedistance feature, DME, and a separate TACAN azimuth feature that provides militarypilots data similar to the civilian VOR). A co-located VOR and TACAN beacon is calleda VORTAC. A VOR with co-located DME only is called a VOR-DME. A VOR radialwith DME distance allows a one-station position fix. Both VOR-DMEs and TACANsshare the same DME system.
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VORTACs and VOR-DMEs use a standardized scheme of VOR frequency -TACAN channel pairing so that a specific VOR frequency is always paired with aspecific channel for the co-located TACAN or DME feature; on civilian equipment, theVHF frequency is tuned and the appropriate TACAN channel is automatically selected.

Some VORs have a relatively small geographic area protected from interferenceby other stations on the same frequency -- called "terminal" or T-VORs. Other stationsmay have protection out to 130 nautical miles (NM) or more. Although it is popularlythought that there is a standard difference in power output between T-VORs and otherstations, in fact the stations' power output is set to provide adequate signal strength in thespecific site's service volume.

How to use a VOR

If a pilot wants to approach the VOR station from due east then the aircraft will

have to fly due west to reach the station. The pilot will use the OBS to rotate the compassdial until the number 27 (270 degrees) aligns with the pointer (called the Primary Index)at the top of the dial. When the aircraft intercepts the 90-degree radial (due east of theVOR station) the needle will be centered and the To/From indicator will show "To".Notice that the pilot set the VOR to indicate the reciprocal; the aircraft will follow the 90-degree radial while the VOR indicates that the course "to" the VOR station is 270degrees. This is called "proceeding inbound on the 090 radial." The pilot needs only tokeep the needle centered to follow the course to the VOR station. If the needle drifts off-center the aircraft would be turned towards the needle until it is centered again. After theaircraft passes over the VOR station the To/From indicator will indicate "From" and theaircraft is then proceeding outbound on the 270 degree radial. The CDI needle may

oscillate or go to full scale in the "cone of confusion" directly over the station but willrecenter once the aircraft has flown a short distance beyond the station.

In the illustration above, notice that the heading ring is set with 254 degrees at theprimary index, the needle is centered and the To/From indicator is showing "From" (FR).The VOR is indicating that the aircraft is on the 254 degree radial, west-southwest "from"the VOR station. If the To/From indicator were showing "To" it would mean the aircraftwas on the 74-degree radial and the course "to" the VOR station was 254 degrees. Notethat there is absolutely no indication of what direction the aircraft is flying. The aircraftcould be flying due north and this snapshot of the VOR could be the moment when itcrossed the 254 degree radial.

Accuracy

The predictable accuracy of the VOR system is 1.4. However, test data indicatethat 99.94% of the time a VOR system has less than 0.35 of error. Internal monitoringof a VOR station will shut it down if the station error exceeds 1.0.[1]
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ARINC 711-10 January 30, 2002 states that receiver accuracy should be within 0.4degrees with a statistical probability of 95% under various conditions. Any receivercompliant to this standard should meet or exceed these tolerances.

Distance measuring equipment (DME)

Distance measuring equipment (DME) is a transponder-based radio navigationtechnology that measures distance by timing the propagation delay ofVHF orUHF radiosignals.

It was invented by Edward George "Taffy" Bowen whilst employed as Chief ofthe Division of Radiophysics of the Commonwealth Scientific and Industrial ResearchOrganisation (CSIRO) in Australia. Another Australian world-first, engineered version ofthe system was deployed by Amalgamated Wireless Australasia Limited in the early1950s operating in the 200 MHz VHF band. This Australian domestic version wasreferred by the Federal Department of Civil Aviation as DME(D) (or DME Domestic),

and the later international version adopted by ICAO as DME(I).

DME is similar tosecondary radar, except in reverse. The system was a post-wardevelopment of the IFF (identification friend or foe) systems of World War II. Tomaintain compatibility, DME is functionally identical to the distance measuringcomponent ofTACAN.

Operation

Aircraft use DME to determine their distance from a land-based transponder by

sending and receiving pulse pairs - two pulses of fixed duration and separation. Theground stations are typically collocated with VORs. A typical DME ground transpondersystem for enroute or terminal navigation will have a 1 kW peak pulse output on theassigned UHF channel.

A low-power DME can also be colocated with an ILS localizerwhere it providesan accurate distance function, similar to that otherwise provided by ILS Marker Beacons.

Hardware

The DME system is composed of a UHF transmitter/receiver (interrogator) in theaircraft and a UHF receiver/transmitter (transponder) on the ground.

Timing

The aircraft interrogates the ground transponder with a series of pulse-pairs(interrogations), The ground station replies with an identical sequence of reply pulse-
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pairs with a precise time delay (typically 50 microseconds). The DME receiver in theaircraft searches for pulse-pairs (X-mode= 12 microsecond spacing) with the correct timeinterval between them. The correct time between pulse pairs is determined by eachindividual aircraft's particular interrogation pattern. The aircraft interrogator locks on tothe DME ground station once it understands that the particular pulse sequence is the

interrogation sequence it sent out originally. Once the receiver is locked on, it has anarrower window in which to look for the echoes and can retain lock.

Distance calculation

A radio pulse takes around 12.36 microseconds to travel one nautical mile to andfrom, this is also referred to as a radar-mile. The time difference between interrogationand reply minus the 50 microsecond ground transponder delay is measured by theinterrogator's timing circuitry and translated into a distance measurement in nauticalmiles which is then displayed in the cockpit.

Specification

A typical DME transponder can provide concurrent distance information to about100 aircraft.[1] Above this limit the transponder avoids overload by limiting the gain ofthe receiver. Replies to weaker more distant interrogations are ignored to lower thetransponder load.

Radio frequency and modulation data

DME frequencies are paired to VHF omnidirectional range (VOR) frequencies. ADME interrogator is designed to automatically tune to the corresponding frequency whenthe associated VOR is selected. An airplanes DME interrogator uses frequencies from1025 to 1150 MHz. DME transponders transmit on a channel in the 962 to 1150 MHzrange and receive on a corresponding channel between 962 to 1213 MHz. The band isdivided into 126channels for interrogation and 126 channels for transponder replies. Theinterrogation and reply frequencies always differ by 63 MHz. The spacing of all channelsis 1 MHz with a signal spectrum width of 100 kHz.

Technical references to X and Y channels relate only to the spacing of theindividual pulses in the DME pulse pair, 12 microsecond spacing for X channels and 36microsecond spacing for Y channels.

DME facilities identify themselves with a 1350 Hz morse code three letteridentity. If collocated with a VOR or ILS it will have the same identity code as the parentfacility. Additionally, the DME will identify itself between those of the parent facility.DME identity is 1350 Hz to differentiate itself from the 1020 Hz tone of the VOR or theILS localizer.
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Accuracy

Accuracy of DME is 185 m (0.1 nm).[1] One important thing to understand is thatDME provides the physical distance from the aircraft to the DME transponder. Thisdistance is often referred to as 'slant range' and depends trigonometrically upon both thealtitude above the transponder and the ground distance from it.

For example, an aircraft directly above the DME station at 6000 feet altitudewould still show one mile on the DME readout. The aircraft technically is a mile away,just a mile straight up. Slant range error is most pronounced at high altitudes when closeto the DME station.
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Tactical Air Navigation (TACAN)

TACAN is a tactical air navigation system for the military services ashore andafloat and TACAN is the military counterpart of civil VHF Omnidirectional Range /Distance Measuring Equipment (VOR/DME). It provides the user with a distance andbearing from a ground station. It is a more accurate version of the VOR/DME system thatprovides range and bearing information forcivil aviation. The azimuth service ofTACAN primarily serves military users. TACAN is limited to line of sight,approximately 180 miles at higher altitudes, because of propagation characteristics andradiated power. Special consideration must be given to the location of ground-basedTACAN facilities, especially in mountainous terrain due to line-of-sight coverage.

TACAN is a UHF radionavigation system that provides bearing and distanceinformation through collocated azimuth and DME antennas. When TACAN is collocatedwith VOR, it is a VORTAC facility. DME and the distance measuring function ofTACAN are functionally the same. At VORTAC facilities, the DME portion of theTACAN system is available for civil use.

The VORTAC facility is a station composed of a VOR for civil bearinginformation and a TACAN for military bearing information and military/civil distancemeasuring information. The TACAN transponder performs the function of a DMEwithout the need for a separate, collocated DME. Because the rotation of the antennacreates a large portion of the azimuth signal, if the antenna fails, the azimuth component

is no longer available and the TACAN downgrades to a DME only mode.

Because the azimuth and range units are combined in one system it provides forsimpler installation. Less space is required than a VOR because a VOR requires a largecounterpoise and a fairly complex phased antenna system. A TACAN systemtheoretically might be placed on a building, a large truck, an airplane, or a ship, and beoperational in a short period of time. TACAN, for example, are used on air refuelingtankers.

For military usage a primary drawback is lack of the ability to control emissions(EMCON) and stealth. Naval TACAN operations are designed so an aircraft can find the

ship and land. There is no encryption involved, an enemy can simply use the range andbearing provided to attack a ship equipped with a TACAN. Some TACANs have theability to employ a "Demand Only" mode wherein they will only transmit wheninterrogated by an aircraft on-channel. It is likely that TACAN will be replaced with adifferential GPS system similar to the Local Area Augmentation System called JPALS.The Joint Precision Approach and Landing System has a low probability of intercept toprevent enemy detection and an aircraft carrier version can be used for autolandoperations.
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Some systems used in the United States modulate the transmitted signal by usinga 900 RPM rotating antenna. Since this antenna is fairly large and must rotate 24 hours aday, it can cause reliability issues. Modern systems have antennas that use electronicrotation (instead of mechanical rotation) and have no moving parts.

A. Signal Characteristics

TACAN is a short-range UHF (962-1215 MHz Aeronautical Radio NavigationService band) radionavigation system designed primarily for military aircraft use.TACAN transmitters and responders provide the data necessary to determine magneticbearing and distance from and aircraft to a selected station.

B. Accuracy

- Predictable - The ground stations errors are less than 1.0 degree for azimuth for

the 135h Hz element and 4.5 degrees for the 15 Hz element. Distance errors are thesame as DME errors.- Relative - The major relative errors emanate from course selection, receiver and

flight technical error.- Repeatable - Major error components of the ground station and receiver will not

vary greatly in the short term. The repeatable error will consist mainly of the flighttechnical error.

C. Availability

A TACAN station can be expected to be available 98 percent of the time

D. Coverage

Most aeronautical radionavigation aids that provide positive course guidance havea designated standard service volume (SSV) that defines the unrestricted reception limitsusable for random or unpublished route navigation. Within the SSV, the Navaid signal isfrequency protected and is available at the altitudes and radial distances indicted in thetable below. In addition to these SSVs, it is possible to define a non-standard servicevolume if siting constraints result in different coverage. SSV limitations do not apply topublished Instrument Flight Rule (IFR) routes and procedures.

Reception below 1,000 feet above ground level is governed by line-of-sightconsiderations, and is described in Section 1-1-8 of the FAA Aeronautical InformationManual. Complete functional and performance characteristics are described in FAAOrder 9840.1, U.S. National Aviation Standard for the VOR/DME/TACAN Systems.

Reception within the SSV is restricted by vertical angle coverage limitations.Distance information from DME and TACAN, and azimuth information from VOR, isnormally usable from the radio horizon to elevation angles of at least 40 degrees. At


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higher elevation angles - within the so-called cone of ambiguity - the Navaid informationmay not be usable.

VOR/DME/TACAN Standard Service Volumes (SSV)

SSV Class Designator Altitude and Range BoundariesT (Terminal) From 1,000 feet above ground level (AGL) up to and including12,000 feet AGL at radial distances out to 25nm.

L (Low Altitude) From 1,000 feet AGL up to and including 18,000 feet AGL at radialdistances out to 40nm

H (High Altitude) From 1,000 feet AGL up to and including 14,500 feet AGL at radialdistances out to 40nm. From 14,500 AGL up to and including60,000 feet at radial distances out to 100nm. From 18,000 feetAGL up to and including 45,000 feet AGL at radial distances out to130nm.

E. Reliability

A TACAN station can be expected to be reliable 98 percent of the time.Unreliable stations, as determined by remote monitors, are automatically removed fromservice

F. Fix Rate

TACAN provides a continuous update of the deviation from a selected course.Initialization is less than one minute after turn on. Actual update rate varies with thedesign of airborne equipment and system loading.

G. Fix Dimensions

The system shows magnetic bearing, deviation in degrees and distance to theTACAN station in nautical miles.

H. System Capacity

For distance information, 110 interrogators are considered reasonable for presenttraffic handling. Future traffic handling could be increased when necessary throughreduced airborne interrogation rates and increased replay rates. Capacity for the azimuthfunction is unlimited.

I. Ambiguity

There is no ambiguity in the TACAN range information. There is a slightprobability of azimuth at multiples of 40 degrees.

J. IntegrityTACAN provides system integrity by removing a signal from use within ten

seconds of an out-of-tolerance condition detected by an independent monitor.

K. Spectrum


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TACAN operates in the 960-1027, 1033-1087, and 1093-1215 MHz sub-bands ofthe 960-1215 MHz ARNS frequency band. It shares those sub-bands with DME

Traffic Collision Avoidance System (TCAS)

The Traffic alert and Collision Avoidance System (or TCAS) is an aircraftcollision avoidance system designed to reduce the incidence of mid-air collisionsbetween aircraft. It monitors the airspace around an aircraft for other aircraft equippedwith a corresponding active transponder, independent of air traffic control, and warnspilots of the presence of other transponder-equipped aircraft which may present a threatofmid-air collision (MAC). It is an implementation of the Airborne Collision AvoidanceSystem mandated byInternational Civil Aviation Organization to be fitted to all aircraftwith MTOM (maximum take-off mass) over 5700 kg or authorised to carry more than 19passengers.

Official definition from PANS-ATM (Nov 2007): ACAS / TCAS is an aircraft

system based onsecondary surveillance radar(SSR) transponder signals which operatesindependently of ground-based equipment to provide advice to the pilot on potentialconflicting aircraft that are equipped with SSR transponders.

In modern glass cockpit aircraft, the TCAS display may be integrated in theNavigation Display (ND); in older glass cockpit aircraft and those with mechanicalinstrumentation, such an integrated TCAS display may replace the mechanicalInstantaneous Vertical Speed Indicator (which indicates the rate with which the aircraft isdescending or climbing).

TCAS basics

TCAS involves communication between all aircraft equipped with an appropriatetransponder (provided the transponder is enabled and set up properly). Each TCAS-equipped aircraft "interrogates" all other aircraft in a determined range about theirposition (via the 1030 MHz radio frequency), and all other craft reply to otherinterrogations (via 1090 MHz). This interrogation-and-response cycle may occur severaltimes per second.

Through this constant back-and-forth communication, the TCAS system builds athree dimensional map of aircraft in the airspace, incorporating their bearing, altitude and

range. Then, by extrapolatingcurrent range and altitude difference to anticipated futurevalues, it determines if a potential collision threat exists.

TCAS and its variants are only able to interact with aircraft that have a correctlyoperating transponder. A unique 24-bit identifier is assigned to each aircraft uponmanufacture and is entered into the transponder. These identifiers can be decoded onlineusing tools such as those at Airframes.org.
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The next step beyond identifying potential collisions is automatically negotiatinga mutual avoidance maneuver (currently, maneuvers are restricted to changes in altitudeand modification of climb/sink rates) between the two (or more) conflicting aircraft.These avoidance maneuvers are communicated to the flight crew by a cockpit display andby synthesized voice instructions.

Safety aspects of TCAS

Safety studies on TCAS estimate that the system improves safety in the airspaceby a factor of between 3 and 5.

However, it is well understood that part of the remaining risk is that TCAS mayinduce midair collisions: "In particular, it is dependent on the accuracy of the threataircrafts reported altitude and on the expectation that the threat aircraft will not make anabrupt maneuver that defeats the TCAS RA. The safety study also shows that TCAS II

will induce some critical near midair collisions..." (See page 7 of Introduction to TCAS IIVersion 7 (PDF) in external links below).

One potential problem with TCAS II is the possibility that a recommendedavoidance maneuver might direct the flight crew to descend toward terrain below a safealtitude. Recent requirements for incorporation of ground proximity mitigate this risk.Ground proximity warning alerts have priority in the cockpit over TCAS alerts.

Some pilots have been unsure how to act when their aircraft was requested toclimb whilst flying at their maximum altitude. The accepted procedure is to follow theclimb RA as best as possible, temporarily trading speed for height. The climb RA shouldquickly finish. In the event of a stall warning, the stall warning would take priority.

Relationship to Automatic Dependent Surveillance (ADS)

Automatic Dependent Surveillance-Broadcast (ADS-B) messages are transmittedfrom aircraft equipped with suitable transponders, containing information such asidentity, location, and velocity. The signals are broadcast on the 1090 MHz radiofrequency. ADS-B messages are also carried on a Universal Access Transceiver (UAT)in the 900 MHz band.

TCAS equipment which is capable of processing ADS-B messages may use thisinformation to enhance the performance of TCAS, using techniques known as "hybridsurveillance". As currently implemented, hybrid surveillance uses reception of ADS-Bmessages from an aircraft to reduce the rate at which the TCAS equipment interrogatesthat aircraft. This reduction in interrogations reduces the use of the 1030/1090 MHz radiochannel, and will over time extend the operationally useful life of TCAS technology. TheADS-B messages will also allow low cost (for aircraft) technology to provide real time
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traffic in the cockpit for small aircraft. Currently UAT based traffic uplinks are providedin Alaska and in regions of the East coast of the USA.

Hybrid surveillance does not include the use any of the aircraft flight informationin the TCAS conflict detection algorithms; ADS-B is used only to identify aircraft thatcan safely be interrogated at a lower rate.

In the future, prediction capabilities may be improved by using the state vectorinformation present in ADS-B messages. Also, since ADS-B messages can be received atgreater range than TCAS normally operates, aircraft can be acquired earlier by the TCAStracking algorithms.

The identity information present in ADS-B messages can be used to label otheraircraft on the cockpit display (where present), improving situational awareness.

Drawbacks to TCAS and ADS-B

The major demonstrated problem of the ADS-B protocol integration is this addedverbosity of the extra information transmitted, which is considered unnecessary forcollision avoidance purposes. The more data transmitted from one aircraft in accordancewith the system design, the lesser the number of aircraft that can participate in thesystem, due to the fixed and limited channel data bandwidth (1 megabit/second with the26/64 data bits to packet length bit capacity of the Mode S downlink data format packet).For every Mode S message of 64 bits, the overhead demands 8 for clock sync at thereceiver and Mode S packet discovery, 6 for type of Mode S packet, 24 for who it camefrom. Since that leaves only 26 for information, multiple packets must be used to conveya single message. The ADS-B "fix" proposal is to go to a 128 bit packet, which is not anaccepted international standard. Either approach increases channel traffic above the levelsustainable for environments such as the Los Angeles Basin.[citation needed]

Versions of TCAS

A. Passive

Collision Avoidance systems which rely on transponder replies triggered byground and airborne systems are considered passive. Ground and airborne interrogatorsquery nearby transpondersfor mode C altitude information, which can be monitored bythird-party systems for traffic information. Passive systems display traffic similar toTCAS, however generally have a range of less than 7 nautical miles (13 km). PortableCollision Avoidance System.

B. TCAS I

TCAS I is the first generation of collision avoidance technology. It is cheaper butless capable than the modern TCAS II system, and is mainly intended forgeneral aviation
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use. TCAS I systems are able to monitor the traffic situation around a plane (to a range ofabout 40 miles) and offer information on the approximate bearing and altitude of otheraircraft. It can also generate collision warnings in the form of a "Traffic Advisory" (TA).The TA warns the pilot that another aircraft is in near vicinity, announcing "traffic,traffic", but does not offer any suggested remedy; it is up to the pilot to decide what to

do, usually with the assistance of Air Traffic Control. When a threat has passed, thesystem announces "clear of conflict".

C. TCAS II

TCAS II is the second and current generation of TCAS, used in the majority ofcommercial aviation aircraft (see table below). It offers all the benefits of TCAS I, butwill also offer the pilot direct, vocalized instructions to avoid danger, known as a"Resolution Advisory" (RA). The suggestive action may be "corrective", suggesting thepilot change vertical speed by announcing, "descend, descend", "climb, climb"or"AdjustVertical Speed Adjust" (meaning reduce or increase vertical speed). By contrast a

"preventive" RA may be issued which simply warns the pilots not to deviate from theirpresent vertical speed, announcing, "monitor vertical speed"or"maintain vertical speed".TCAS II systems coordinate their resolution advisories before issuing commands to thepilots, so that if one aircraft is instructed to descend, the other will typically be told toclimb maximising the separation between the two aircraft.

As of 2006, the only implementation that meets the ACAS II standards set byICAO is Version 7.0 of TCAS II, produced by three avionics manufacturers: RockwellCollins, Honeywell, and ACSS (Aviation Communication & Surveillance Systems; an L-3 Communications andThales Avionics company).

D. TCAS III

TCAS III was the "next generation" of collision avoidance technology whichunderwent development by aviation companies such as Honeywell. TCAS IIIincorporated technical upgrades to the TCAS II system, and had the capability to offertraffic advisories and resolve traffic conflicts using horizontal as well as verticalmanouevring directives to pilots. For instance, in a head-on situation, one aircraft mightbe directed, "turn right, climb" while the other would be directed "turn right, descend."This would act to further increase the total separation between aircraft, in both horizontaland vertical aspects. All work on TCAS III is currently suspended and there are no plansfor its implementation.[1]

needed.[2]

TCAS Limitations

While the benefits of TCAS are undisputable, it can be assumed that TCAS' truetechnical and operational potential (and thus its possible benefits) is not yet being fullyexploited because of the following limitations in current implementations:
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TCAS is limited to supporting only vertical separation advisories ATC isn't automatically informed about resolution advisories issued by TCAS-so

that controllers may be unaware of TCAS-based resolution advisories or evenissue conflicting instructions (unless ATC is explicitly informed about an issuedRA during a high-workload situation), which may be a source of confusion for the

affected crews In the above context, TCAS lacks automated facilities to enable pilots to easily

report and acknowledge reception of a (mandatory) RA to ATC (and intention tocomply with it), so that voice radio is currently the only option to do so, whichhowever additionally increases pilot workload

Today's TCAS displays do not provide information about resolution advisoriesissued to other (conflicting) aircraft, while resolution advisories issued to otheraircraft may seem irrelevant to another aircraft, this information would enable andhelp crews to assess whether other aircraft (conflicting traffic) actually complywith RAs by comparing the actual rate of (altitude) change with the requested rateof change (which could be done automatically and visualized accordingly),

thereby providing crucial realtime information for situational awareness duringhighly critical situations TCAS equipment today is often primarily range-based, as such it only displays

the traffic situation within a configurable range of miles/feet, however undercertain circumstances a "time-based" representation (i.e. within the next xxminutes) might be more intuitive.

Lack of terrain/ground awareness information, which might be critical forcreating feasible (non-dangerous, in the context of terrain clearance) and usefulresolution advisories (i.e. prevent extreme descent instructions if close to terrain),to ensure that TCAS RAs never facilitate CFIT scenarios.

Aircraft performance in general and current performance capabilities in particular(due to active aircraft configuration) are not taken into account during thenegotiation and creation of resolution advisories (as it is the case for differencesbetween different types of aircraft, e.g. turboprop/jet vs. helicopters), so that it istheoretically possible that resolution advisories are issued that demand climb orsink rates outside the normal/safe flight envelope of an aircraft during a certainphase of flight (i.e. due to the aircraft's current configuration), furthermore alltraffic is being dealt with equally, there's basically no distinction taking placebetween different types of aircraft, neglecting the option of possibly exploitingaircraft-specific (performance) information to issue customized and optimizedinstructions for any given traffic conflict (i.e. by issuing climb instructions tothose aircraft that can provide the best climb rates, while issuing descendinstructions to aircraft providing comparatively better sink rates, therebyhopefully maximizing altitude change per time unit, that is separation)

TCAS is primarily extrapolation-oriented, as such it is using algorithms trying toapproximate 4D trajectory prediction, in order to assess and evaluate the currenttraffic situation within an aircraft's proximity, however the degree of data-reliability and usefulness could be significantly improved by enhancing saidinformation with limited access to relevant flight plan information, as well as torelevant ATC instructions to get a more comprehensive picture of other traffic's


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(route) plans and intentions, so that flight path predictions would no longer bemerely based on estimations but rather aircraft routing (FMSflight plan) and ATCinstructions.

For TCAS to work effectively, it needs to be fitted to all aircraft in a givenairspace. However, TCAS is not fitted to many smaller aircraft mainly due to the

high costs involved (between $25,000 and $150,000). Many smaller personalbusiness jets for example, are currently not legally required to have TCASinstalled, even though they fly in the same airspace as larger aircraft that arerequired to have proper TCAS equipment on board.
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Radio Magnetic Indicator (RMI)

A Radio-Magnetic Indicator (RMI) is an

instrument which displays aircraft heading andbearing to selected radio navigation aids. It is alsoan alternate ADF display that provides moreinformation than a standard ADF. While the ADFshows relative angle of the transmitter withrespect to the aircraft, an RMI displayincorporates a compass card, actuated by theaircraft's compass system, and permits theoperator to read the magnetic bearing to or fromthe transmitting station, without resorting toarithmetic.

Most RMI incorporate two direction needles. Often one needle (generally the thin,single-barred needle) is connected to an ADF and the other (thicker and/or double-barred)is connected to a VOR. Using multiple indicators a navigator can accurately fix theposition of their aircraft without requiring station passage.


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Ground Proximity Warning System (GPWS) /

Enhanced Ground Proximity Warning System

(EGPWS)

Ground Proximity Warning System (GPWS) is asystem designed to alert pilots if their aircraft is inimmediate danger of flying into the ground. Anothercommon name for such a system is Ground-CollisionWarning System (GCWS).

Don Bateman, a Canadian born engineer is creditedwith the invention of GPWS.

The system monitors an aircraft's height above ground as determined by radioaltimeter. A computer then keeps track of these readings, calculates trends, and will warnthe captain with visual and audio messages if the aircraft is in certain defined flyingconfigurations ("modes").

The modes are:

Excessive descent rate ("PULL UP" "SINKRATE")Excessive terrain closure rate ("TERRAIN" "PULL UP")Altitude loss after take off ("DON'T SINK")

Unsafe terrain clearance ("TOO LOW - TERRAIN" "TOO LOW - GEAR" "TOO LOW -FLAPS")Excessive deviation below glideslope ("GLIDESLOPE")

Traditional GPWS does have a blind spot. Since it can only gather data fromdirectly below the aircraft, it must predict future terrain features. If there is a dramaticchange in terrain, such as a steep slope, GPWS will not detect the aircraft closure rateuntil it is too late for evasive action.

A new technology, the Enhanced Ground Proximity Warning System (EGPWS)

solves this problem by combining a worldwide digital terrain database with a Long-Range Navigation System such as Global Positioning System, INS (Inertial NavigationSystem), Radio-Dependant navigational systems, or a combination of the above. On-board computers can compare its current location with a database of the Earth's terrain.Pilots will receive much more timely cautions and warnings of any obstructions to theaircraft's path.


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The Black Box

A. Flight data recorder

History

The first prototype FDR was produced in 1957 by Dr David Warren of the thenAeronautical Research Laboratories of Australia. In 1953 and 1954, a series of fatalmishaps on the de Havilland DH106 Comet prompted the grounding of the entire fleetpending an investigation. Dr Warren, a chemist specializing in aircraft fuels, wasinvolved in a professional committee discussing the possible causes. Since there had beenno witnesses, and no survivors, Dr Warren began to conceive of a crash survivablemethod to record the flight crew's conversation, reasoning they would likely know thecause. Despite his 1954 report entitled "A Device for Assisting Investigation into AircraftAccidents" and a 1957 prototype FDR named "The ARL Flight Memory Unit", aviation

authorities from around the world were largely uninterested. This changed in 1958 whenSir Robert Hardingham, the Secretary of the UK Air Registration Board, becameinterested. Dr Warren was asked to create a pre-production model which culminated intothe "Red Egg", the world's first commercial FDR by the British firm, S. Davall & Son.The "Red Egg" got its name from the shape and bright red color. Incidentally, the term"Black Box" came from a meeting about the "Red Egg", when afterwards a journalist toldDr Warren, "This is a wonderful black box."

FDR Design

Modern FDRs are typically double wrapped, in strong corrosion-resistant stainlesssteel or titanium, with high-temperature insulation inside. Although they are called"black boxes," aviation recorders are actually painted bright orange.

In the early days, data were embossed onto a type of magnetic foil known asIncanol Steel. The foil proved to be destructible and FDR manufacturers began using amore reliable form of magnetic tape. Electromagnetic technology remained the data-recording medium of choice until the late 1990s, when solid-state electronics began toshow promise. Solid-state recorders rely on stacked arrays of non-moveable memorychips. The technology is considered more reliable than magnetic tape, as the lack ofmoving parts provides a reduced chance of breakage during a crash.

Solid-state recorders are considered much more reliable than their magnetic-tapecounterparts. Solid state uses stacked arrays ofmemory chips, so they don't have movingparts. With no moving parts, there are fewer maintenance issues and a decreased chanceof something breaking during a crash.

Data from both the CVR and FDR is stored on stacked memory boards inside thecrash-survivable memory unit (CSMU). The stacked memory boards are about 1.75
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inches (4.45 cm) in diameter and 1 inch (2.54 cm) tall. Memory boards have enoughdigital storage space to accommodate two hours of audio data for CVRs and 25 hours offlight data for FDRs.

The black box is powered by one of two power generators that draw their power

from theplane's engines. One generator is a 28-volt DC power source, and the other is a115-volt, 400-hertz (Hz) AC power source

FDRDesign Legal Standards

The design of today's FDR is governed by the internationally recognisedstandards and recommended practices relating to flight recorders which are contained inICAO Annex 6 which makes reference to industry crashworthiness and fire protectionspecifications such as those to be found in the European Organisation for Civil AviationEquipment documents EUROCAE ED55, ED56A and ED112 (Minimum Operational

Performance Specification for Crash Protected Airborne Recorder Systems). In theUnited States, the Federal Aviation Administration (FAA) regulates all aspects of U.S.aviation, and cites design requirements in their Technical Standard Order, based on theEUROCAE documents (as do the aviation authorities of many other countries).

Currently, EUROCAE specifies that a recorder must be able to withstand anacceleration of 3400g(33 km/s) for 6.5 milliseconds. This is roughly equivalent to animpactvelocity of 270 knots and a deceleration or crushing distance of 450 cm.Additionally, there are requirements for penetration resistance, static crush, high and lowtemperature fires, deep sea pressure, sea waterimmersion, and fluid immersion.

Crash - Survivable Memory Unit

In many airline accidents, the only devices that survive are the crash-survivablememory units (CSMUs) of the fli

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